by Catalina Camia, USA TODAY

by Catalina Camia, USA TODAY

Sen. Joe Manchin of West Virginia is a Democrat who likes to say he's not tied to President Obama.

A new study by Congressional Quarterly magazine finds Manchin, a former governor who was elected to a full Senate term in November, opposed his party's standard-bearer more often than any other Democratic senator. Manchin opposed Obama on 2012 votes more than 15% of the time, according to CQ's vote study.

Among Obama's fellow Democrats, Sen. Mary Landrieu of Louisiana is the only one currently serving who opposed the president at least 10% of the time last year. She came in third behind Manchin and then Virginia's Jim Webb, who retired.

Wayne Parent, a political scientist at Louisiana State University, suggests Landrieu's voting record won't go unnoticed when she is on the ballot next year.

"If she can keep her reputation as a 'Louisiana first' senator, she'll likely win again," Parent told USA TODAY. "That reputation depends both on her tenacious work for the state and a bipartisan voting record, which is why her practice of often voting against her party is significant."

One of the Democrats to support Obama the most - and who is up for election in 2014 - is Sen. Tim Johnson of South Dakota. CQ found Johnson supported Obama nearly 99% of the time on issues where the president's position was clear. That puts Johnson up there with Democratic senators such as New York's Charles Schumer in terms of presidential support.

Johnson hasn't said whether he'll run for another term in South Dakota, a state that has been trending Republican. Former governor Mike Rounds, a Republican, has already announced his Senate bid.